[BUGFIX] Fix JS error with Context Sensitive Help

Using the edit form for records in a new pop window leads
to a JS error, as "top" is not defined in the new window.

Resolves: #26588
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I2eab83a2c9df2fed13fd5bcc1c829bf4f2b9a055
Reviewed-on: https://review.typo3.org/21548
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters